1. Technical Field of the Invention
The present invention relates to the field of integrated circuits, and more particularly to mask-programmed read-only memory (mask-ROM).
2. Prior Arts
For a mask-programmed read-only memory (mask-ROM), content is written using at least one data-mask during manufacturing process (step 10 of FIG. 1C). For example, a data-mask 2 comprises a plurality of mask-regions 2a-2i, whose patterns represent content data 4a-4i (FIG. 1A). Hereinafter, the pattern representing content data is referred to as data-pattern. Being permanently formed, the data-patterns cannot be modified once written onto the data-mask 2.
In prior art, whenever a new content needs to be included in a mask-ROM, a new data-mask 2x needs to be made to replace the original data-mask 2 (step 12 of FIG. 1C). For example, the new data-mask 2x includes the data-pattern of the new content 4e* in the mask-region 2e (FIG. 1B), as well as the data-patterns for the original contents 4a-4d, 4f-4i. The original and new contents 4a-4d, 4e*, 4f-4i are written to a second batch of mask-ROMs using the new data-mask 2x (step 14 of FIG. 1C).
As technology advances, data-mask becomes more and more expensive. For example, a 22 nm data-mask costs ˜$260 k. In addition, a data-mask contains more and more data. For example, a 22 nm data-mask contains up to ˜155 GB data. Some of these data will likely be revised at a future point of time. Replacing a whole data-mask for small data revision is costly. To overcome this and other drawbacks, the present invention discloses a mask-ROM with reserved space (mask-ROMRS).